In general, mixture gas of hydrogen and carbon monoxide, i.e. synthetic gas is an important medium in synthesizing an environmentally clean fuel and a chemical raw material such as ammonia, methanol, acetic acid, dimethyl ether (DME), synthetic gasoline and diesel, and various mole rations (H2/CO) of hydrogen and carbon monoxide are needed to synthesize such products. For example, a mole ratio of 2/1 is needed for synthesizing methanol, a mole ratio of 1/1 is needed for synthesizing acetic acid, methyl formate or DME.
The synthetic gas has been produced from coal, petroleum, natural gas, biomass and even from organic waste of an organic compound. The natural gas is increasing in use since it is not only the largest amount of source for producing the synthetic gas at present but also the most inexpensive and environment-friendly.
As a technique of using the natural gas to produce the synthetic gas, there are steam reforming using methane (wet reforming), partial-oxidation using methane, reforming using carbon-dioxide (dry reforming), and a combination of the steam reforming using methane and the reforming using carbon-dioxide. Among them, the steam reforming using methane is a traditional and potential industrial process for producing the synthetic gas.
This method is generally called the wet reforming, and a mole ratio of hydrogen/carbon monoxide has to be 3 or higher in the wet reforming Thus, the wet reforming is proper to synthesize ammonia, but needs extra hydrogen in processes for synthesizing methanol and the like. In a wet process reaction, at least 1 mole of methane is required in producing 1 mole of carbon monoxide.CH4+H2O→CO+3H2ΔH=206 kJ/molCH4+CO2→2CO+2H2ΔH=247 kJ/mol
The CH4—CO2 reforming is a very attractive process for producing the synthetic gas since it uses carbon dioxide and requires less methane.
In comparison with the wet-reforming process and the partial-oxidation processes, the CH4—CO2 reforming method stoichiometrically needs ½ mole of methane to produce 1 mole of carbon monoxide since carbon dioxide is also a carbon source. The CH4—CO2 reforming has a mole ratio of hydrogen/carbon monoxide is 1/1, but it is relatively easy to control the mole ratio of hydrogen/carbon monoxide by adjusting the ratio of methane/carbon dioxide in a feeding process. Therefore, the synthetic gas produced by the CH4—CO2 reforming is not only utilizable in a process of producing acetic acid or methyl formate, but also satisfies the mole ratio of hydrogen/carbon monoxide needed for producing various substances when it is combined with the wet process.
However, the CH4—CO2 reforming process is a high endothermic reaction, and needs special methods for achieving a considerable reaction rate in order to satisfy conditions required in industry. In this context, catalyst and plasma techniques have not been commercialized hitherto even though they are regarded as potential techniques for satisfying the conditions required in the industry.
Referring to FIGS. 1 and 2, in the CH4—CO2 catalyst reforming process, methane and carbon dioxide are injected to a tubiform fixed-bed reactor filled with a catalyst in a catalyst reaction process, and thermal energy needed for the reaction is supplied by combustion energy of natural gas from an outside of a reactor. Although a CH4—CO2 catalyst reforming reactor can be used together with a wet reforming reactor using methane, carbon deposited on a surface of a catalyst and inactivating the catalyst is a major obstacle to change of the CH4—CO2 catalyst reforming process from a laboratory scale to a commercialization scale.
In addition, a plasma CH4—CO2 reforming process was performed by arc discharge under very limited conditions. In comparison with the catalyst reforming process, the plasma CH4—CO2 reforming reaction having an electromagnetic induction chemical reaction and a thermochemical reaction shows a high conversion rate and selectivity and has no problem of the carbon deposition. Accordingly, despite energy used for generating plasma, the plasma CH4—CO2 reforming process has been continuously researched and attracted attention over the past decade.